In the related art, large-scale three-dimensional curved surfaces, such as ship hull plate or the like, have been mostly formed by line heating. Although forming by line heating has been performed by skilled workers using their experience and intuition, the lack of productive capacities is growing with the aging of such workers.
Accordingly, research has been progressing in order to seek automation of the forming of three-dimensional curved surfaces, and in regard to the forming of small curvature surfaces, automation of forming by line heating has already been successful. In this method, straight-line heating tests for each heating condition (e.g. specification of a coil, excitation frequency, current, voltage, moving speed of a coil, or the like) are performed, inherent strains are classified and put into a database, and heating lines are arranged based on the analysis using the database. In forming small curvature surfaces, the heating lines are largely-spaced, and thus respective heating units can follow the above-described method without interfering with one another (See Non Patent Document 1).
However, in forming large-curvature surfaces, the heating lines may be densely-arranged, the same place may be repeatedly-heated, or the heating lines may cross each other. Also, since a non-straight line heating is frequently used, the generated inherent strains differ even if the heating conditions are the same.
Accordingly, even if the inherent strains according to the heating conditions of the respective heating lines are overlapped by using the database, the resultant inherent strains may differ from the actually generated inherent strains. Accordingly, if the heating lines are arranged based on the inherent strains identified by the straight-line heating test, the working accuracy deteriorates beyond the permissible limit.
That is, the inherent strains that are generated during large-curvature surface forming process (e.g. under the conditions such as narrow gaps between the heating lines, the repeated heating of the heating lines, crossing of the heating lines, non-straight heating lines, or the like) are different from the inherent strains from the straight-line heating test, and have not yet been identified. Accordingly, the automation of the forming of the large-curvature surfaces has not been achieved.
[Non Patent Document 1] Ishiyama et al., “Automatic line-heating bending process method applying a finite element method (FEM)”, Manual of Ishikawa-jima Harima 1999 Vol. 39 No. 2 p. 60-p. 64